add modifier to png image?
As you have read from the other answers this is not a good way to do this. I would use actual 3D text like they did in the tutorial you posted. 

However to give your question an answer. 




Import image as planes (choose your text PNG)
Make sure you subdivide your text plane to give it more geometry to bend. 
Make an Empty and leave it in the center of your text. 
on your text/image object add the simple deform modifier and choose the empty as the object to bend around. 


Keep in mind you are very limited this way because you are using a picture of text and not actual text.